1. Field of the Invention
The present invention relates to range switching devices that perform switching among drive, reverse, and neutral ranges in an automatic transmission of a vehicle such as an automobile, and more particularly to a range switching device that prevents reverse running and ensures safety even in the case of a failure.
2. Description of the Related Art
In an automatic transmission such as a CVT or a planetary gear step AT provided in an automobile or the like, the drive, reverse, and neutral ranges are switched by controlling the oil pressure supplied to engagement elements such as a forward clutch and a reverse clutch.
Switching among such drive, reverse, and neutral ranges has conventionally been performed by a manual valve connected by a mechanical linkage to an operation lever operated by the driver.
Recently, there has been proposed a technology so-called shift-by-wire system in which switching of running ranges is performed only by electric signals, without providing a mechanical linkage between the operation lever and the transmission.
As an example of a conventional technique relating to shift-by-wire systems of automatic transmission, Japanese Unexamined Patent Application Publication (JP-A) No. 2008-128475 describes a range switching device in which spool valves are actuated by three solenoid valves to switch the oil pressure supplied to drive and reverse hydraulic servos.
Further, JP-A No. 2008-128473 describes a range switching device in which running ranges are switched by two solenoid valves and the running range can be maintained even when either one of the solenoid valves fails in the running range.
However, with the technique described in JP-A No. 2008-128475, when switching between drive and reverse is performed using a drive/reverse switching valve, where a failure mode occurs such that the state of the valve is reversed, abrupt switching can take place from drive to reverse or from reverse to drive.
To resolve this problem, it is possible to increase the failsafe ability by detecting the failure state and creating different combinations of control states of a plurality of valves, but in this case where the failure detection speed is low, the designed combination is not obtained and the vehicle might run in reverse.